NiO/NiWO4 Composite Yolk-Shell Spheres with Nanoscale NiO Outer Layer for Ultrasensitive and Selective Detection of Subppm-level p-Xylene

Fabricating Co9S8@NiWO4 Electrode toward Record Area Capacitance of 18.8 F cm-2 for High Performance Supercapacitor

Realizing ultrahigh area capacitance based on suitable electrodes is a challenge for supercapacitors. In this work, an electrode is designed consisting of Co9S8 nanotubes and NiWO4 nanoparticles (named Co9S8@NiWO4) and fabricated by a hydrothermal process. The Co9S8@NiWO4 electrode realizes a record area capacitance of 18.8 F cm-2 at a current density of 5 mA cm-2 among the reports in the literature. The corresponding asymmetric supercapacitor shows high energy density of 0.48 mW h cm-2 at 2.42 mW cm-2 and superior cyclic stability of 79.2% retention over 7000 cycles at 10 mA cm-2. The great promotion in performance of Co9S8@NiWO4 electrode is synergistically attributed to high intrinsic theoretical capacitances of compounds, abundance polyvalent states, improved conductivity, and increases in reaction sites and rate of the electrode. This work provides a new route for designing highly efficient electrode for developing supercapacitors with high performance in practical applications.

Nickel Oxide Nanostructures for Gas Sensing: Recent Advances, Challenges, and Future Perspectives

The need for efficient and reliable gas sensors has grown significantly due to increasing industrial activities, transportation, and environmental pollution, posing serious risks to human health and the environment. Advanced sensor technologies are crucial for detecting these harmful gases at low concentrations with a high accuracy. Nickel oxide, a p-type metal oxide semiconductor, has emerged as a promising candidate for gas sensing applications owing to its unique and excellent structural, electronic, and catalytic properties along with its high chemical stability. Interestingly, the possibility to synthesize NiO in versatile nanostructure forms: nanowires, nanoflowers, and nanospheres, helps to enhance surface area and porosity, which are critical factors to improve gas adsorption and diffusion. This review presents a comprehensive and critical assessment of the latest advancements in the synthesis, characterization, and gas-sensing performance of NiO nanostructures. We explore how structural modifications, such as decoration with noble metal nanoparticles, formation of different composites, and surface functionalization with self-assembly enhance the sensitivity, selectivity, and operational temperature of NiO sensors. Particular focus is given to the integration of NiO in novel nanoheterostructures, where the formation of p-n and p-p junctions significantly improves charge transport and overall sensor response. Finally, we identify current challenges in reproducibility, stability, and operating conditions, while offering directions for future research on tailoring NiO nanostructures for more effective, scalable, and robust sensor technologies.

Multiple-Yolk-Shell NiO Microspheres for Selective Detection of m-Xylene

m-Xylene is a volatile organic compound that is extensively used in various industrial processes. It is toxic, posing significant risks to human health and the environment. Therefore, developing gas sensors with high sensitivity and selectivity for m-xylene detection is critical. In this work, we demonstrated the synthesis of NiO-yolk double-shell (NiO-YDS) and NiO-yolk triple-shell (NiO-YTS) derived from NiO/Ni-BTC and NiO/Ni-PTA composites, respectively, using the microwave-assisted solvothermal method from Ni-BTC-derived NiO spheres. The NiO/Ni-BTC composite has trimesic acid (H3BTC) as an organic linker, while NiO/Ni-PTA has p-terephthalic acid (PTA). We investigated the sensing properties of these materials for 2-butanone, 2-nonanone, 3-methyl-1-butanol, acetone, benzene, ethanol, methanol, and m-xylene. These composites exhibited excellent sensitivity and selectivity for detecting m-xylene under dry conditions. Specifically, the NiO-YTS sensor showed a sensitivity of 217.5% to m-xylene, while the NiO-YDS sensor demonstrated a sensitivity of 179.8% at 350 °C in dry air. We emphasize the NiO-YTS composite due to its superior sensitivity and selectivity in detecting m-xylene compared with the NiO-YDS composite. The NiO-YTS sensor exhibited stable and reproducible sensing performance for 100 ppm of m-xylene under optimum working conditions, with a theoretical detection limit of 5.43 ppb and relatively fast response time (89 s) and recovery time (191 s). This work describes an easy method for synthesizing NiO-YDS and NiO-YTS derived from NiO/Ni-BTC and NiO/Ni-PTA composites. It demonstrates that these composites represent a new class of materials that can potentially enhance the sensitivity and selectivity of m-xylene gas sensors.

Constructing magnetic/dielectric dual loss and phonon/electron thermal carriers γ-Al2O3-based yolk-shell microspheres to collaboratively advance microwave absorption and heat conduction

The severe electromagnetic (EM) interference and overheating issues in 5G/6G electric devices increasingly heighten the need for developing multifunctional materials with large heat conduction (HC) and high EM wave (EMW) absorption. Here, a series of γ-Al2O3-based yolk-shell microspheres (γ-AlOOH, γ-Al2O3, γ-Al2O3@C, γ-Al2O3@Fe3O4@C, and γ-Al2O3@FeAl2O4@Fe@C YSMSs) as multifunctional fillers are investigated for the simultaneous improvement in the HC and EMW absorption of γ-Al2O3-based composites. Using γ-AlOOH YSMSs as precursors produced from a hydrothermal method, the γ-Al2O3-based YSMSs were synthesized via an annealing route or soaking-annealing route; their phases, textures, and compositions were finely adjusted by changing the Al3+/Fe3+ molar ratio (β) and annealing temperature (Ta). Results show that the thermal transfers in the γ-Al2O3-based YSMSs are promoted by the synergic effect of phonons and electrons when they are utilized as thermally conductive fillers. Comparatively, the γ-Al2O3@FeAl2O4@Fe@C YSMSs formed at β = 8 : 2 and Ta = 700 °C exhibit a high HC of 1.84-3.29 W m-1 K-1 in a loading amount of 5-40%, exceeding those of not merely γ-Al2O3, γ-AlOOH, γ-Al2O3@C, and γ-Al2O3@Fe3O4@C YSMSs but also most previously reported fillers. Furthermore, the γ-Al2O3@Fe3O4@C YSMSs exhibit prominent EMW absorption properties with a large ABW/d of 4.49 GHz mm-1 (just 30% loading), superior to most other Al2O3-based absorbers. Such excellent EMW absorption could be explained by magnetic/dielectric dual loss and significant cavity and interfacial effects caused by yolk-shell structures. In conclusion, this work inspires the development of yolk-shell structures with magnetic/dielectric dual loss and phonon/electron thermal carriers as high-performance bifunctional materials with exceptional heat conduction and EMW absorption.

Mesoporous-Structure MOF-14-Based QCM p-Xylene Gas Sensor

In this work, a facile synthesis method was adopted to synthesize MOF-14 with mesoporous structure. The physical properties of the samples were characterized by PXRD, FESEM, TEM and FT-IR spectrometry. By coating the mesoporous-structure MOF-14 on the surface of a quartz crystal microbalance (QCM), the fabricated gravimetric sensor exhibits high sensitivity to p-toluene vapor even at trace levels. Additionally, the limit of detection (LOD) of the sensor obtained experimentally is lower than 100 ppb, and the theoretical detection limit is 57 ppb. Furthermore, good gas selectivity and fast response (15 s) and recovery (20 s) abilities are also illustrated along with high sensitivity. These sensing data indicate the excellent performance of the fabricated mesoporous-structure MOF-14-based p-xylene QCM sensor. On the basis of temperature-varying experiments, an adsorption enthalpy of -59.88 kJ/mol was obtained, implying the existence of moderate and reversible chemisorption between MOF-14 and p-xylene molecules. This is the crucial factor that endows MOF-14 with exceptional p-xylene-sensing abilities. This work has proved that MOF materials such as MOF-14 are promising in gravimetric-type gas-sensing applications and worthy of future study.

Highly Sensitive and Selective Real-Time Breath Isoprene Detection using the Gas Reforming Reaction of MOF-Derived Nanoreactors

Real-time breath isoprene sensing provides noninvasive methods for monitoring human metabolism and early diagnosis of cardiovascular diseases. Nonetheless, the stable alkene structure and high humidity of the breath hinder sensitive and selective isoprene detection. In this work, we derived well-defined Co₃O₄@polyoxometalate yolk-shell structures using a metal-organic framework template. The inner space, including highly catalytic Co₃O₄ yolks surrounded by a semipermeable polyoxometalate shell, enables stable isoprene to be reformed to reactive intermediate species by increasing the gas residence time and the reaction with the inner catalyst. This sensor exhibited selective isoprene detection with an extremely high chemiresistive response (180.6) and low detection limit (0.58 ppb). The high sensing performance can be attributed to electronic sensitization and catalytic promotion effects. In addition, the reforming reaction of isoprene is further confirmed by the proton transfer reaction-quadrupole mass spectrometry analysis. The practical feasibility of this sensor in smart healthcare applications is exhibited by monitoring muscle activity during the workout.

Designing oxide chemiresistors for detecting volatile aromatic compounds: recent progresses and future perspectives

Oxide chemiresistors have mostly been used to detect reactive gases such as ethanol, acetone, formaldehyde, nitric dioxide, and carbon monoxide. However, the selective and sensitive detection of volatile aromatic compounds such as benzene, toluene, and xylene, which are extremely toxic and harmful, using oxide chemiresistors remains challenging because of the molecular stability of benzene rings containing chemicals. Moreover, the performance of the sensing materials is insufficient to detect trace concentration levels of volatile aromatic compounds, which lead to harmful effects on human beings. Here, the strategies for designing highly selective and sensitive volatile aromatic compound gas sensors using oxide chemiresistors were suggested and reviewed. Key approaches include the use of thermal activation, design of sensing materials with high catalytic activity, the utilization of catalytic microreactors and bilayer structures with catalytic overlayer, and the pretreatment of analyte gases or post analysis of sensing signals. In addition, future perspectives from the viewpoint of designing sensing materials and sensor structures for high-performance and robust volatile aromatic compounds gas sensors are provided. Finally, we discuss possible applications of the sensors and sensor arrays.

Recent Progress of Nanostructured Sensing Materials from 0D to 3D: Overview of Structure-Property-Application Relationship for Gas Sensors

Along with the progress of nanoscience and nanotechnology, nanomaterials with attractive structural and functional properties have gained more attention than ever before, especially in the field of electronic sensors. In recent years, the gas sensing devices have made great achievement and also created wide application prospects, which leads to a new wave of research for designing advanced sensing materials. There is no doubt that the characteristics are highly governed by the sensitive layers. For this reason, important advances for the outstanding, novel sensing materials with different dimensional structures including 0D, 1D, 2D, and 3D are reported and summarized systematically. The sensing materials cover noble metals, metal oxide semiconductors, carbon nanomaterials, metal dichalcogenides, g-C₃ N₄ , MXenes, and complex composites. Discussion is also extended to the relation between sensing performances and their structure, electronic properties, and surface chemistry. In addition, some gas sensing related applications are also highlighted, including environment monitoring, breath analysis, food quality and safety, and flexible wearable electronics, from current situation and the facing challenges to the future research perspectives.

Ultrahigh-sensitive mixed-potential ammonia sensor using dual-functional NiWO4 electrocatalyst for exhaust environment monitoring

The exhaust monitoring for in-situ quantification of gas pollutants has always been a challenge due to the harsh thermo-chemical environments, for which the solid-electrolyte based gas sensors appear as a realistic solution. In this work, an ultrahigh-sensitive mixed-potential ammonia sensor was developed using a new dual-functional NiWO₄ electrocatalyst, synthesized through a low-temperature molten-salt synthesis route. The electrode morphology and diffusion lengths were tuned for optimum performance. The sensor operated at 550 ℃ displayed response of -100 mV to 80 ppm NH₃, with response/recovery times of 28/68 s and a record-high sensitivity of 90 mV/decade. Besides, it displayed excellent selectivity and trace-level NH₃ detection ability upto 400 ppb. While examining the sensing mechanism, the sensor exhibited an NH₃ concentration-dependent transformation of rate-determining kinetics from charge-transfer limited Butler-Volmer type to diffusional mass-transport limited reaction kinetics. Moreover, the remarkable long-term stability with negligible response degradation (< 4%) confirms the suitability of the sensor for exhaust environment monitoring.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

Yolk-shell (Cu,Zn)Fe2O4 ferrite nano-microspheres with highly selective triethylamine gas-sensing properties

Multicomponent spinel ferrites are essential to be used in high-performance gas-sensing materials. Herein, multinary (Cu,Zn)Fe2O4 spinel nano-microspheres with tunable internal structures, including solid, core-shell, and yolk-shell, were successfully synthesized by a simple self-templated solvothermal method combined with a subsequent annealing strategy. The internal structures of the (Cu,Zn)Fe2O4 nano-microspheres significantly rely on the heating rates of the precursors, which show promising selective response towards trimethylamine gas. Among them, the as-formed yolk-shell (Cu,Zn)Fe2O4 nano-microspheres exhibited high response to triethylamine with excellent selectivity of STEA/SX = 1.86 at 160 °C, fast response-recovery rate (58 s/136 s), and long-term repeatability and stability of more than one month. The corresponding triethylamine gas-sensing mechanism with the special microstructures is discussed. This work provides new insights into the rational design of interior structure and the modulation of high gas response and selectivity of multinary spinel ferrites in gas-sensing applications.

Advances in the synthesis and design of nanostructured materials by aerosol spray processes for efficient energy storage

The increasing demand for energy storage has motivated the search for highly efficient electrode materials for use in rechargeable batteries with enhanced energy density and longer cycle life. One of the most promising strategies for achieving improved battery performance is altering the architecture of nanostructured materials employed as electrode materials in the energy storage field. Among numerous synthetic methods suggested for the fabrication of nanostructured materials, aerosol spray techniques such as spray pyrolysis, spray drying, and flame spray pyrolysis are reliable, as they are facile, cost-effective, and continuous processes that enable the synthesis of nanostructured electrode materials with desired morphologies and compositions with controlled stoichiometry. The post-treatment of spray-processed powders enables the fabrication of oxide, sulfide, and selenide nanostructures hybridized with carbonaceous materials including amorphous carbon, reduced graphene oxide, carbon nanotubes, etc. In this article, recent progress in the synthesis of nanostructured electrode materials by spray processes and their general formation mechanisms are discussed in detail. A brief introduction to the working principles of each spray process is given first, and synthetic strategies for the design of electrode materials for lithium-ion, sodium-ion, lithium-sulfur, lithium-selenium, and lithium-oxygen batteries are discussed along with some examples. This analysis sheds light on the synthesis of nanostructured materials by spray processes and paves the way toward the design of other novel and advanced nanostructured materials for high performance electrodes in rechargeable batteries of the future.

Highly Selective, ppb-Level Xylene Gas Detection by Sn2+-Doped NiO Flower-Like Microspheres Prepared by a One-Step Hydrothermal Method

Detecting xylene gas is an important means of avoiding human harm from gas poisoning. A precise measurement demands that the gas sensor used must have high sensitivity, high selectivity, and low working temperature. To meet these requirements, in this study, Sn²⁺-doped NiO flower-like microspheres (SNM) with different amounts of Sn²⁺ synthesized by a one-step hydrothermal process were investigated. The responses of gas sensors based on different Sn²⁺-doped NiO materials for various targeting gases were fully characterized. It was found that all of the synthesized materials exhibited the best gas response at a working temperature of 180 degrees, which was much lower than the previously reported working temperature range of 300–500 degrees. When exposed to 10 ppm xylene, the 8 at% Sn²⁺-doped NiO sensor (mol ratio) exhibited the highest response, with a value of 30 (R_g/R_a). More significantly, the detection limit of the 8 at% Sn²⁺-doped NiO sensor for xylene is down in the ppb level. The Sn²⁺-doped NiO material also exhibits excellent selectivity for other gases with long-term stability and repeatability. The significant improvement in the response to xylene can theoretically be attributed to a decrease in the intrinsic hole carrier concentration, higher amounts of adsorbed oxygen and active sites.

ZnxCo3−xO4 bimetallic oxides derived from metal–organic frameworks for enhanced acetone sensing performances

Gas sensors based on Zn_xCo_3−xO₄ bimetallic oxides derived from metal–organic frameworks exhibit a very high response of 35.6 to acetone and the limit of detection is as low as 0.5 ppm.

Selenium vacancies enriched the performance of supercapacitors with excellent cycling stability via a simple chemical bath deposition method

Herein, we report a simple and cost-effective route for the fabrication of NiWO₄, NiWO₄P, and NiWO₄Se nanostructures using the chemical bath deposition method.

Coordination Polymer-Derived Multishelled Mixed Ni-Co Oxide Microspheres for Robust and Selective Detection of Xylene

Multishell, stable, porous metal-oxide microspheres (Ni-Co oxides, Co₃O₄ and NiO) have been synthesized through the amorphous coordination polymer-based self-templated method. Both oxides of Ni and Co show poor selectivity to xylene, but the composite phase has substantial selectivity (e.g., S_xylene/ S_ethanol = 2.69) and remarkable sensitivity (11.5-5 ppm xylene at 255 °C). The short response and recovery times (6 and 9 s), excellent humidity-resistance performance (with coefficient of variation = 11.4%), good cyclability, and long-term stability (sensitivity attenuation of ∼9.5% after 30 days and stable sensitivity thereafter) all show that this composite is a competitive solution to the problem of xylene sensing. The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.

Nanostructured Ternary Metal Tungstate-Based Photocatalysts for Environmental Purification and Solar Water Splitting: A Review

Visible-light-responsive ternary metal tungstate (MWO₄) photocatalysts are being increasingly investigated for energy conversion and environmental purification applications owing to their striking features, including low cost, eco-friendliness, and high stability under acidic and oxidative conditions. However, rapid recombination of photoinduced electron-hole pairs and a narrow light response range to the solar spectrum lead to low photocatalytic activity of MWO₄-based materials, thus significantly hampering their wide usage in practice. To enable their widespread practical usage, significant efforts have been devoted, by developing new concepts and innovative strategies. In this review, we aim to provide an integrated overview of the fundamentals and recent progress of MWO₄-based photocatalysts. Furthermore, different strategies, including morphological control, surface modification, heteroatom doping, and heterojunction fabrication, which are employed to promote the photocatalytic activities of MWO₄-based materials, are systematically summarized and discussed. Finally, existing challenges and a future perspective are also provided to shed light on the development of highly efficient MWO₄-based photocatalysts.

Aliovalent Fe(iii)-doped NiO microspheres for enhanced butanol gas sensing properties

Fe-Doped NiO multi-shelled microspheres have been synthesized via a facile hydrothermal reaction.